overbank sediments: a natural bed blending sampling medium ... metal and rbf... · sediments...
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Chemometrics and Intelligent Laborat
Overbank sediments:
a natural bed blending sampling medium for large—scale
geochemical mappingB
B. Bblvikena,*, J. Bogenb, M. Jartuna, M. Langedalc, R.T. Ottesena, T. Voldena
aGeological Survey of Norway, NO-7491 Trondheim, NorwaybNorwegian Water Resources and Energy Administration, P.O. Box 5091 Majorstua, NO-0301 Oslo, Norway
cCity of Trondheim, NO-7004 Trondheim, Norway
Received 15 December 2003; received in revised form 28 May 2004; accepted 17 June 2004
Available online 15 September 2004
Abstract
Overbank sediments occur along rivers and streams with variable water discharge. They are deposited on floodplains and levees from
water suspension during floods, when the discharge exceeds the amounts that can be contained within the normal channel. Overbank
sediments were introduced as a sampling medium in geochemical mapping in 1989, and a number of studies have later been published on this
subject. These papers indicate:
1. Depth integrated samples of overbank sediments reflect the composition of many current and past sediment sources upstream of the
sampling point, contrary to active stream sediments, which originate in a more restricted number of presently active sediment sources
from which they move regularly along the stream channel. In many regions overbank sediments are more representative of drainage
basins than active stream sediments and can, therefore, be used to determine main regional to continental geochemical distribution
patterns with widely scattered sample sites at low cost per unit area.
2. Samples of overbank sediments can be collected in floodplains or old terraces along laterally stable or slowly migrating channels. In
some locations the surface sediments may be polluted, however, natural, pre-industrial sediments may, nevertheless, occur at depth.
Mapping of the composition of recent and pre-industrial overbank sediments can, therefore, be used (i) in a characterization of the
present state of pollution, and (ii) as a regional prospecting tool in natural as well as polluted environments.3. Vertical movements of elements in strata of overbank sediments may occur, especially in cases where the distribution of relatively mobile
elements in non-calcareous areas are heavily influenced by acid rain. However, the overall impression is that vertical migration of
chemical elements is not a major problem in the use of overbank sediments in geochemical mapping.4. The composition of overbank sediment is of great interest to society in general, since flood plains are very important for agriculture,
urbanisation, and as sources for drinking water.
Several of the above points indicate that overbank sediments represent a natural analogue to the products of bed-blending. This aspect is
mentioned here in light of the Theory of Sampling (TOS).
D 2004 Elsevier B.V. All rights reserved.
0169-7439/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemolab.2004.06.006
B Please note that the figures in this article appear in full colour in the
online version on www.sciencedirect.com.* Corresponding author. Tel.: + 47 6116 4709; fax: +47 6116 8236.
E-mail address: [email protected].
1. Introduction
Geochemical mapping includes (1) systematic sampling
of natural materials, such as rocks, sediments, soils and
waters; (2) chemical analysis of the samples; and (3)
illustration of the analytical results on maps. Geochemical
ory Systems 74 (2004) 183–199
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199184
maps show significant natural distribution patterns with great
contrasts. Such distributions occur for many chemical
elements at various scales from local up to continental. This
property of geochemical data implies that geochemical maps
are of great interest to society in general, especially in fields
such as (1) environmental research, (2) exploration of mineral
deposits, (3) medical geology (geomedicine), (4) agriculture
and (5) land use planning. These aspects are discussed below.
Fig. 1. Lead content (mg/kg) in stream sediments from the Gal3a river andits tributaries, Hedmark county, Norway. After Bjbrlykke et al. [5].
1. The use of geochemical maps in environmental research
is based on the fact that pollution implies addition to the
environment of any substance at a rate that results in
higher than natural concentrations of that substance [1].
Consequently, data on the spatial variations in the
composition of uncontaminated natural materials are a
necessary prerequisite for an evaluation of the degree of
pollution within small and large areas. Geochemical
maps based on chemical analysis of natural materials
provide such information.
2. In exploration, geochemical maps may disclose geo-
chemical provinces or geochemical anomalies with
greater than normal contents of heavy metals or other
elements of economic interest. Follow-up investigations
in such environments may lead to the discovery of
workable deposits.
3. It is widely accepted that some local or regional
environments may be sub-optimal for the health of
human beings and other animals. Relations between
goitre and iodine deficiency and between caries and
fluorine deficiency are classical examples of this kind.
Geochemical maps provide information for research in
this emerging field of geomedicine, which is also called
medical geology.
4. In agriculture, information about variations in the
chemical composition of soils may be utilized in
production planning, since some chemical elements are
vital for plants and domestic animals, while others may
be harmful when present in too high concentrations.
5. In land-use planning, geochemical maps may contribute
to information about the suitability of specific areas for
specific uses.
It was previously assumed that the composition of active
sediments regularly moving along the stream channel
represents the geochemistry of large parts of the drainage
basin upstream of the sample site. As a consequence,
regional geochemical maps were often based on analysis of
active sediments collected at certain intervals along streams
of high order (usually 1–20 km2 catchments) [2].
In 1989 Ottesen et al. [3] reviewed this procedure and
claimed that active sediments in stream channels do not
reflect the chemical composition of large parts of drainage
areas, since they often originate in discrete sources of limited
extension. They suggested, however, that overbank sediment
would be a more representative type of sample. Many papers
have since been published concerning the use of overbank
sediment as a geochemical sampling medium. Our contribu-
tion summarizes the main results demonstrated in these
papers with emphasis on representativity and sampling errors
(reproducibility), which are also important aspects of the
Theory of Sampling (TOS) [4].
In order to put these results into perspective, we give three
examples of published geochemical maps at various scales, a
short summary about sampling density in geochemical
mapping and some comments on problems that may occur
in the use of chemical analysis in geochemical mapping.
2. Geochemical mapping
2.1. Examples of geochemical maps at various scales
The literature has many examples of geochemical maps
at scales ranging from local to continental. Three examples
of such maps are shown here in order to present the type of
data that may be obtained by geochemical mapping.
Fig. 1 shows a geochemical Pb anomaly in active stream
sediments disclosed during a mineral exploration project in
Southern Norway [5]. The anomaly comprises Pb contents
of 270–680 mg/kg, which is much higher than the
background levels in the surrounding 30 km2 (10–20 mg
Pb/kg). Follow-up investigations led to the discovery of an
earlier unknown Pb deposit. Even though the deposit later
proved to be sub-economic, the example shows that
geochemical mapping is a potential prospecting tool.
Fig. 2 is a geochemical map of Cr reproduced from the
geochemical atlas of Northern Fennoscandia [6]. The atlas
contains 136 single-point geochemical maps at a scale of
1:4,000,000 based on the contents of total and acid
extractable elements in four different types of sample (till,
active stream sediments, stream organic matter and stream
moss) collected at 5000–6000 sites within an area of
250,000 km2 (1 sample station per 50 km2). Systematic
distribution patterns were obtained for most elements. The
maps for the contents of an element in various sample types
are with only few exceptions similar—even after using
different chemical digestion methods on the original
samples or heavy mineral fractions of composites of
ø
Fig. 2. Chromium content in stream sediments from Northern Fennoscandia. After Bblviken et al. [6].
B.Bølviken
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B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199186
geographical neighbours. This indicates that the consisten-
cies and thus the reliabilities of the maps are good. The
maps, which were originally obtained for use in mineral
exploration, are also of great interest in other fields, such as
environmental research.
Fig. 3 shows the distribution of K in the conterminous
USA. The map is based on chemical analysis of 1300 soil
samples collected across the entire country [7]. In spite of the
low sampling density (1 sample per 5000 km2), systematic
distribution patterns are disclosed for this and also for several
other elements. Taking the sampling density into account, the
reliability of the K pattern appears to be acceptable when
compared with a map of K acquired by airborne radiometric
surveys, which include millions of measurements [8], (see
Figs. 4 and 5). Some of the obtained geochemical patterns
coincide with known geological structures, while others
indicate structures, which had not previously been recog-
nized. The maps have an interesting potential use in mineral
exploration, environmental research and epidemiology (geo-
medicine, medical geology).
2.2. Sampling density in relation to survey area
The number of samples per unit area (sampling density) in
geochemical mapping has been a matter of controversy
between geochemists for many years. It has, for example,
been claimed that less than 1 sample per 25 km2 is insufficient
Fig. 3. Potassium in surface soil from the conterminous USA. The map is obtai
samples spread over the area. After Gustavsson et al. [7].
in regional geochemical mapping as the geochemical patterns
become distorted at lower sampling densities [9]. The present
authors think, however, that empirical data contradict this
viewpoint. The Li-maps in Fig. 6 were obtained in the
Nordkalott Project [6]. The original map, which is based on
nearly 6000 samples of stream sediments within a survey area
of 250,000 km2 (1 sample per 40 km2), shows systematic
distribution patterns. Moreover, most of this general pattern is
maintained in random selections down to approximately 25%
of the total number of samples. In this case 1 sample per 160
km2 appears to be a lower limit below which the geochemical
pattern may become distorted.
However, successful application of even much more
scattered sampling has been reported in the literature. The
lowest sampling density known to the authors is that used in
Northern Europe by Eden and Bjfrklund [10]. They
collected 49 samples of each of till, active stream sediments
and overbank sediments from a survey area of 1.1 million
km2 (1 sample site per 23,000 km2). Nevertheless, system-
atic patterns were obtained for the contents of several
elements in various sample types. Several of these patterns
agree with those obtained at much denser sampling, see also
the comments on p. 9.
The examples of geochemical maps given in the present
paper, as well as earlier statistical treatment of regional
geochemical data [11], point to the interesting possibility
that geochemical distributions are scale-independent, i.e.
ned by calculating the moving average (N=50) between results from 1300
Fig. 4. Contents of potassium at the surface of the conterminous USA derived from aerial gamma-ray surveys. After Duval et al. [8].
Fig. 5. Moving values for Spearman-rank correlation coefficients (N=50) between the contents of potassium in surface soils and potassium determined by air-
born gamma-ray surveys. After Gustavsson et al. [7].
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 187
Fig. 6. Contents of acid soluble lithium in 5773 samples (100%) and random selections of 50% and 25 % of the original samples collected during the
Nordkalott Project, see Bblviken et al. [6].
Fig. 7. Example of an unpublished map from the Nordkalott project,
Northern Fennoscandia [6]. The patterns are not real showing only effects
of annual variations in analytical bias for the contents of W in stream moss.
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199188
have fractal properties (e.g. Refs. [12,13]). If this is true, the
notion of a minimal sampling density of general use
becomes meaningless. Fractal properties would indicate
that a feasible sampling density will depend on the size of
the survey area; the larger the survey area, the lower an
acceptable sampling density would be.
If the goal is to determine the main geochemical patterns
within a certain area, an optimal sample number may exist
independent of the size of the area. The examples above
suggest that a reasonable sample number would be in the
order of 1000, even at a continental scale. Such a small
number of samples would keep costs low, but require that
the composition of each of the collected samples is typical
for the sub-area it represents, in other words, it must be
representative in the sense of TOS. This points to the use of
overbank sediment from large streams as a potential
sampling medium in regional to even global geochemical
mapping.
2.3. Chemical analysis in geochemical mapping
A number of excellent methods of chemical analysis are
now available for geochemical mapping. For details, the
reader is referred to special publications, e.g. Fletcher [14].
Here we stress only two aspects, namely (1) reproducibility
and quality control, and (2) the significance of determining
the total or an extractable fraction of the elements.
(1) It is generally supposed that in applied geochemistry,
sampling errors are normally greater than the errors of
chemical analysis. This feature, which agrees with con-
clusions drawn from TOS, may be true for random
analytical errors. However, experience has also shown that
in geochemical mapping a small analytical bias may
sometimes be serious. Fig. 7 is an example taken from the
raw data of the Nordkalott project, Northern Fennoscandia
[6]. The map shows a region of low W-contents in the
central part of the survey area. This region has rectilinear
north–south striking borders juxtaposed against regions of
high W-concentrations in the eastern and western parts.
Similar patterns were not obtained for any other element.
One team did all the sampling, and all the chemical data
were obtained with the same method of analysis at the same
laboratory. Nevertheless, the patterns on the map are not
real, but reflect only variations in the analytical bias
between three different years of analysis. This map was,
therefore, not published.
The authors are aware of several examples of similar
results in other projects, where a small analytical bias has
caused apparently significant spatial patterns, because the
samples have been analysed in the geographical order of
sampling. This experience has led us to the following
conclusion: Geochemical mapping procedures require a
thorough system for quality control. In such a system
reference samples should be included at random within the
collection of samples for analysis. Ideally, all real and
reference samples should be analysed in random order and
in one batch after all the sampling has been completed.
(2) Dissolution of mineralogical samples in acids and
other solvents varies with the composition of the samples.
Minerals with low contents of Si tend to be more soluble
than those with high contents of Si. Consequently, for some
chemical elements (e.g. Mg) similar spatial patterns are
Fig. 8. Contents of total and acid soluble potassium in overbank sediments, Norway. After Ottesen et al. [26].
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 189
obtained for the total contents as for an acid soluble fraction,
while for other elements (e.g. K) the patterns for totals are
different from those of the acid soluble fraction, see Fig 8.
Many environmental surveys focus on the use of easily
extractable forms of chemical elements, since they may be
closer to a biologically available fraction than the total
concentrations.
3. Overbank sediments as a representative sampling
medium in geochemical mapping
Since Ottesen et al. [3] advocated that overbank sedi-
ments could be a potentially more representative sample
type than active stream sediments, many publications have
appeared concerning the use of overbank sediments in
Fig. 9. Water discharges during ordinary conditions with norma
geochemical mapping. The following section summarizes
the main aspects of these contributions.
3.1. Formation of overbank sediments
Overbank sediments (also called alluvial soils, floodplain
sediments or levee sediments) occur along streams with
variable water discharge. In flooding streams, the tempora-
rily enhanced discharge may exceed the amounts that can be
contained within the normal channel (Fig. 9). Material in
suspension will then be transported onto floodplains and
levees, where it may be laid down and accumulated,
especially during the latest phases of flooding. In most
streams, this process has taken place many times in the past.
Overbank deposits, therefore, consist of successive nearly
horizontal strata of young sediments overlaying older
l amounts of water (A, left), and major floods (B, right).
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199190
sediments. A vertical section through such a deposit reflects
the history of sedimentation back through time.
During floods, the great quantity of water activates many
sediments sources in the drainage area, and the material in
suspension will reflect the composition of these and earlier
developed sources. This is a reason why a composite sample
consisting of many layers of overbank sediments would
represent large parts of—or even complete—catchments.
In most cases, deposits of overbank sediment contain
mainly natural material throughout. However, in some
situations anthropogenic material may have polluted the
most recent layers. The composition of sediments at depth
may, nevertheless, still be natural.
In some occurrences of overbank sediments, the stratig-
raphy may be complex due to re-deposition of material
eroded from earlier formed upstream deposits. Young
sediments may then be intermixed with older sediments.
Various aspects of such situations are examined in the
section on sampling errors below.
3.2. Sampling errors
Since overbank sediments normally consist of individual
horizontal strata formed at different times, the variations in
chemical composition and the corresponding sampling error
would be greater in the vertical than in the horizontal
direction.
3.2.1. Vertical variations in the chemical composition of
overbank sediments
In principle vertical trends in the chemical composition
of overbank sediments have two origins, namely variations
in the composition of the original source material, and
alterations caused by secondary migration of substances
after deposition.
The first trend is caused by time variations in the
positions of the most active sediment sources as well as the
intensity of flooding, while the second trend is due to
features such as climate, pH, reduction/oxidation conditions,
amounts and type of organic material, biological activities
and time. These last factors are the same as those found to
govern the dispersion of elements in soils and lake
sediments (e.g. Refs. [15,16,17]). In many climates soil
formation processes may need hundreds of years in order to
develop significant vertical patterns. For overbank sedi-
ments the available time intervals are normally more
restricted, because new sediments are occasionally depos-
ited on top of the older ones. It appears, therefore, that
problems of vertical migration in general would be less in
overbank sediments than in other soils. However, distribu-
tions of mobile elements such as Cd could be exceptions,
because of their high solubility. The following examples
from various countries illustrate effects of these two
principles.
In Belgium, the Netherlands, Luxembourg and parts of
Germany, 34 overbank sediment profiles situated along the
banks of meandering rivers were studied [18–21]. In 30 of
these, pre-industrial sequences could be detected below
polluted surface overbank sediments. Samples were col-
lected at depth intervals of 10 cm and analysed for their
contents of major and trace elements. 14C dating was
performed from all parts of the sections where sufficient
organic material could be obtained.
Three main groups of vertical distribution patterns were
distinguished in the sections, namely (1) either low or high
metal concentrations throughout the profile. This is
thought to reflect a generally low or high natural metal
content in the catchments, (2) no variations in grain size or
lithology, but a gradual increase in heavy metal concen-
trations towards the top of the profile. Such patterns are
presumably caused by airborne pollution, (3) abrupt
changes in metal concentrations at certain depths and a
corresponding change in lithology. These patterns are
interpreted as being an effect of man-made discharges
into the catchments and a subsequent fluvial dispersion of
particle-bound pollutants.
A combination of types 2 and 3 above may occur where
the results of soil-forming processes are superimposed on
the original patterns. Signs of vertical migration of Fe and
Mn were pronounced in some profiles, but correlations
between the contents of Fe or Mn and most other heavy
metals were not found. However, in some profiles vertical
percolation was indicated for mobile elements such as As
and Cd.
Profiles that did not show pre-industrial sediments in the
lower strata, were interpreted as being a sub-group of type 2,
where the profile was not deep enough to obtain pre-
industrial sediments, or the floodplain had been reworked,
so that pre-industrial overbank sediments had been washed
away.
In a Norwegian study [22–25], overbank sediment
profiles were sampled from the Knabe3na-Kvina drainage
basin, which are influenced by Cu and Mo-rich tailings from
the now closed Knaben Molybdenum Mine. Along the
rivers, pre-industrial overbank sediments were detected
below the present inundation level in the bottom sections
of 14 out of 18 profiles. The four atypical profiles are
situated where lateral river migration has had an impact on
the sedimentary environment, or where minor river regu-
lations and influx of tailings have altered the original peat
bog and lacustrine environments into flood plains.
Most profiles show high Cu and Mo contents in the
upper section, while concentrations at depth in the bottom
section approach a lower, probably natural level similar to
those in the natural sediments above the present inundation
zone of polluted sediments (Fig. 10). In some profiles about
30% of the Cu content in the upper layers appears to be
removed by dissolution or cation exchange and re-precipi-
tated in the middle of the section possibly adhering to
organic matter. However, the sharp decrease of Mo
concentrations below the upper layers, indicates that vertical
migration of Mo is negligible.
Fig. 10. Overbank sediment profile at the polluted Knaben3a river, Southern Norway. After Langedal [22–24]. L.O.I.: Loss on ignition.
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 191
In southern Fennoscandia, Eden and Bjfrklund [10], see
also p. 4, suspected downward percolation of long-range
atmospheric pollution to be the cause of high Pb
concentrations in the lower part of overbank sediment
profiles. Acid rain and low buffer capacity in the sediments
may have contributed to the migration. However, Ottesen
et al. [26] questioned this interpretation and claimed that
the patterns of Pb enrichments in southern Norway are
natural.
In English and Welsh basins with old Pb/Zn-mines, the
vertical distributions of Pb and Zn in overbank sediments
were found to be closely related to the mining history,
suggesting that no significant vertical migration of these
metals had taken place after deposition of the sediments
[27]. Similarly, along the rivers Rio Guanajuato and Rio
Puerco, Mexico, no vertical migration was seen for any of
the elements As, Cr, Cu, Pb, Sn, and Zn [28].
Volden et al. [29] collected top and bottom samples of
overbank sediments at 43 sites within a 12,000 km2 area
around the Russian nickel mining and smelting industry on
the Kola Peninsula, and analysed the samples for 30
elements, of which results for Co, Cu and Ni are reported
in Table 1. The median values for these elements in
Table 1
Median and maximum contents (mg/kg, N=43) of aqua regia extractable
metals in top and bottom parts of overbank sediments (b0.125 mm)
compared with median values in soils (b2 mm) from the same catchments
Overbank sediments Soils
Top Bottom O-horizon C-horizon
Med. Max. Med. Max. Med. Med.
Co 5 60 8 94 2 5
Cu 10 595 20 849 13 16
Ni 9 645 17 938 14 11
Data from a 12,000 km2 area surrounding the Russian nickel mining and
smelting industry on the Kola Peninsula. After Volden et al. [29].
overbank sediments compare rather closely with those in
the O- and C-horizons of soil samples taken from the same
catchments. For most other elements, however, the O-
horizon shows concentrations different to those in other
types of sample. In regional geochemical mapping, the
median values for the composition of overbank sediments
appears thus to reflect that of the deeper soil levels, till and
bedrock. It is unclear to which extent very high metal
values in the overbank sediments mirror natural levels or
vertical percolation of elements of anthropogenic origin.
Further studies of this problem are warranted. They require
more detailed sampling of vertical overbank sediment
profiles than the 10 cm spacing used at selected sites in
this study.
Sampling of overbank sediment has also been performed
in connection with archaeological studies in mining areas in
the Hartz mountains, Germany showing that heavy metal
pollution could be detected in overbank sediments at depths
of several meters making a documentation of the natural
background difficult [30].
3.2.2. Lateral natural variations in the composition of
overbank sediment
Within floodplains the natural lateral variations in the
composition of overbank sediment seem to be small.
In a study of 49 selected floodplains across the
Fennoscandian shield, Eden and Bjfrklund [10] found that
the lateral within floodplain variations were insignificant in
relation to the between-floodplain variation, see Table 2.
Similar results were also obtained in parts of Scandinavia by
Chekushin et al. [31] and Pulkinen and Rissanen [32].
Demetriades and Volden [33] studied the reproducibility
of overbank sediment sampling in Greece and Norway, of
which the results for Greece are referred to here. Ten 60–
600 km2 drainage basins distributed over the Eastern
Macedonia and Thrace regions in Greece were selected
for sampling. After excluding the upper 5–10 cm, two
Table 2
Analysis of variance [34] [35] of the contents of aqua regia soluble
chemical elements in widely spaced duplicate samples of overbank
sediments taken at depth and near the surface in a 23,000 km2 area in
Northern Europe [10]
1 2 3 4
% F F F
Al 2.2 14.5 6.1 4.2
Ba 6.0 14.7 5.6 5.4
Ca 1.8 73.0 15.3 15.3
Co 10.0 12.0 5.8 7.8
Cr 4.7 48.0 7.1 8.3
Cu 10.7 34.7 5.6 5.3
Fe 25.5 13.0 5.1 4.4
K 3.8 33.3 10.5 10.8
La 4.3 34.0 9.3 6.1
Mg 1.8 66.0 8.6 10.3
Mn 6.2 14.1 3.7 6.0
Na 7.3 4.2 5.6 4.6
Ni 10.1 26.0 7.3 7.9
P 4.0 25.0 6.3 8.9
Pb 21.7 5.5 3.9 4.1
Sr 5.0 47.2 8.2 8.3
Th 35.8 10.7 3.9 5.6
Ti 2.2 33.3 6.7 6.9
V 4.5 7.0 6.0 5.0
Zn 5.0 30.0 6.2 7.7
Critical F
value at p=0.05
1.7 1.4 1.4
Numbers of pairs 36 36 116 116
(1)–(3) Samples at depth. (1) Combined sampling and analytical error. (2)
Ratio between total variance and combined sampling and analytical
variance. (3) Ratio of between site variance and within site lateral variance.
(4) Ratio of between site variance and within site vertical variance.
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199192
composite samples (A and B) were collected from vertical
sections 60–100 m apart at the down-stream apex of each
drainage basin. These paired samples were analysed chemi-
cally for the total contents of 21 elements.
Statistical treatments [34,35] of the analytical results
show: (1) The Spearman rank correlation coefficients
between the A and the B samples are significant at the
95% level (or better) for all elements (Al, As, Ba, Be, Ca,
Co, Cu, Fe, Li, Mn, Ni, Pb, S, Sc, Sr, Ti, V, Y, and Zn)
except Cd and Mo. For these two elements the sensitivity of
the analytical method was not adequate, and (2) The
majority of the elements (Al, As, Be, Ca, Co, Cu, Fe, Li,
Mn, Ni, Pb, Sb, Sc, Sr, Ti, V, Y) vary significantly between
sites in relation to within-site variations ( pb0.01). However,
the between-site variations for Ba, Cd, Mo and Zn are
insignificant. For Cd and Mo this is ascribed to the
analytical method, while those for Ba and Zn probably are
caused by high sampling variability.
Langedal [24] found that in floodplain surface sediments
(0–25 cm) of the polluted Knabe3na river in Norway, the
highest Cu and Mo concentrations occur in samples near the
river as well as in depressions. Enrichment of metals in
these parts of the floodplains may be an effect of differences
in the timing of the sediment transport pulse, and the timing
of floodplain inundation, see also [36]. In proximal areas
and depressions the suspended sediment transport rates are
often highest during the rising and peak stages. In polluted
streams these are the first to be inundated and receive the
largest load of particle-bound metals. Similar results were
also found along the Geul river, Belgium [37,38].
3.2.3. Complex stratigraphy due to combined vertical and
lateral processes
Complex stratigraphy in deposits of overbank sediments
may cause sampling problems downstream from mines and
other local sources of severe pollution.
According to Lewin and Macklin [39], fluvially trans-
ported mine tailings may be incorporated in alluvial
sediments in three different ways depending on the river
platform stability.
(1) Along single thread, laterally stable channels,
tailings are mainly accumulated as overbank sediments
on the floodplains. Thus, young sediments overlay older
sediments. (2) In single thread, meandering rivers, tailings
are mainly accumulated around point-bars. In this case
floodplain deposits become younger towards the margins
of the channel. As the river migrates, the tailings will be
reworked and the lateral age distribution may be
disturbed. (3) In rivers where mine tailings are discharged
into the channel, the sediment load increases to such an
extent that the alluvial plain is aggrading. The river may
then become laterally unstable with frequent channel
shifts. This results in a complex floodplain stratigraphy.
After mining ceases, erosion in the alluvial plain will
possibly rework both tailings and pre-industrial material.
This may also influence downstream overbank sediment
profiles.
Macklin et al. [27] and Ridgway et al. [28] evaluated the
use of overbank sediments in geochemical mapping within
areas of England, Wales and Mexico polluted by mining
activities. These two papers share the conclusion that
variations in the composition of overbank sediments may
be so complicated that time consuming detailed studies of
the geomorphology, history and ages of the sediments are
required at each sample station in order to distinguish
between natural and polluted patterns. A single overbank
profile very rarely spans the period from before anthro-
pogenic disturbance through the Industrial Revolution and
later. According to these authors such considerations and
associated costs may render overbank sediment non-viable
as a regional mapping medium. Instead they recommend to
use active stream sediments.
Ridgeway et al. [28] also found that in Mexico the lateral
variations of element contents in overbank sediments are
small for natural sediments, but become more complex for
deposits with mixed pristine and polluted sediments.
The conclusions drawn after the studies in Wales and
Mexico have later been questioned by other researchers (see
e.g. Ref. [40]). It is difficult to imagine that active stream
sediments would be superior to overbank sediments in
polluted environments, since active sediments (contrary to
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 193
overbank sediments) are always polluted to an unknown
degree.
3.2.4. Concluding remarks on sampling error
In both small and large catchments, the sampling error
for natural overbank sediments within a floodplain is small
in relation to the between floodplain variation. This
conclusion appears to be valid in most regions of the world
for both genuine natural deposits and in situations where
pristine sediments at depth are covered by polluted surface
sediments.
Sampling of older terraces is appropriate in order to
obtain pre-industrial material. Such sampling should be
done above the present inundation zone to avoid material
draped during recent floods. Along laterally stable river-
reaches, sampling in the bottom sections of the sediment
profiles is also adequate. Sampling along meandering
reaches, as suggested by Bogen et al. [41], may also be a
possibility if the lateral migration is slow.
Pollution of overbank sediments may be of two types: (1)
Mine wastes and other anthropogenic material may enter the
stream from local sources and then be transported down-
stream. (2) Airborne contaminants originating from distant
sources may reach the catchments. Situation (1) is often
recognizable, since the sources may be easily identified.
Situation (2) can be more difficult to recognize straight
away. In both cases (1) and (2) the contaminants may be
confined to the surface layers of the overbank sediments.
However, in some locations mixtures of old natural and
resent polluted sediments may occur, causing an intricate
stratigraphy in the overbank deposits. In addition, down-
ward percolation of soluble surface pollutants may also
contaminate the sediments at depth. In such cases detailed
investigations at each sample site may be necessary.
It is concluded that only trained personnel should be used
in order to select appropriate locations for sampling of
overbank sediments. If this prerequisite is fulfilled, high
quality subsequent chemical analysis of the samples will
produce reliable data for most chemical elements.
3.3. Representativity and regional distribution of chemical
elements in overbank sediments
Many papers have appeared in the last few years
presenting data on the spatial distribution of element
contents in overbank sediments. Selected publications with
examples from eight countries in Asia, Europe and North
America are referred below in chronological order of
appearance.
Ottesen et al. [3] pioneered the use of overbank
sediments in geochemical mapping and edited an atlas of
geochemical maps based on this type of sample [26]. Nearly
700 floodplains distributed across Norway (300,000 km2)
were selected for sampling. Each plain represents drainage
areas of between 60 and 600 km2 . The samples were
collected at distances of 2–200 m from the present-day
stream, depending on local circumstances. Where possible,
sites close to the stream were avoided in order to reduce the
possibility of collecting polluted samples. A vertical section
through the sediment was cut with a spade, and a composite
sample (5 kg) was taken from the section excluding the
upper 5–10 cm. After drying, the samples were sieved to a
minus 0.062 mm fraction, which were subsequently
analysed for the total contents of 30 elements and an acid
soluble fraction of 29 elements.
Most elements show systematic patterns with great
contrasts. In some cases these patterns agree with known
geological structures, in others they indicate structures not
known earlier. Examples of the maps are shown in Fig. 8
(see comments on p. 00) and Fig. 11. The last figure shows
that the contents of acid soluble Mo in Norwegian overbank
sediments are relatively high in most of southern Norway,
while the levels further north vary. The province of high Mo
concentrations in the south agrees with the results of earlier
prospecting, which disclosed a great number of small and
some more extensive Mo-deposits within the province,
including those mined at Knaben (see p. 9 and e.g. Bugge
[42]).
McConnell et al. [43] carried out geochemical mapping
in the Baie Verte/Springdale area of Newfoundland (2000
km2) based on several types of sample media including
overbank and stream sediments. One hundred twenty-one
samples of each of these types were collected from drainage
basins 2–10 km2 in size, sieved to minus 0.063 mm and
analysed for 38 elements. In this survey area overbank
sediments were found to be more widespread and easier to
sample than stream sediments. In general, trace element
distributions are similar in the two media, both producing
significant patterns reflecting the chemistry of the under-
lying bedrock. For most elements the concentrations are
greater in the overbank than in the stream sediments,
supposedly because overbank sediments are more fine-
grained than stream sediments. In some drainages stream
sediments are contaminated by past mining activity, while
overbank sediments, however, appear unaffected.
Eden and Bjfrklund [10] performed ultra-low density
sampling of overbank sediment and other sample types
across Finland, Norway and Sweden (1 sample station per
23,000 km2), and found that for 20 elements the within site
variation is small compared with the between site variation
(Table 2), and that one sample of overbank sediment may
substitute for 6–20 till samples depending on type of
drainage area.
Xiachu and Mingkai [44] performed an orientation
survey in a part (170,000 km2) of the Jiangxi Province of
Southern China in order to develop techniques of
implementing global ultra-low sampling in geochemical
mapping [45]. Sample sites (1 site per 1800 km2) were laid
out at the apexes of 94 drainage basins, the sizes of which
were between 100 and 800 km2. Composite samples of
overbank sediment were collected from the upper (5–40
cm) and the lower (50–120 cm) layers of sediment profiles
Fig. 11. Contents of acid soluble molybdenum in overbank sediment, Norway. After Ottesen et al. [26].
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199194
within terraces located 3–5 m above the average present
stream water level. The samples were analysed for 39
elements.
For each drainage basin the contents of five selected
elements (Cu, Pb, Sn, W and Zn) in the overbank sediments
were compared with the averages of the same elements
obtained in the National Geochemical Mapping Project of
China (1–2 samples per km2), see Fig. 12. There are good
correlations between the concentrations of these elements in
overbank sediments and averages (NN30) for the same
elements computed for earlier obtained samples, see an
example for lead in Fig. 13.
Xiachu and Mingkai [44] concluded that (1) floodplains
of 100–800 km2 catchments are suitable sample stations
for global geochemical mapping based on overbank
sediments, (2) sampling of wide-spaced lower-layer over-
bank sediment is a fast and cost-effective way to identify
geochemical provinces, (3) there is a significant correlation
Fig. 12. Illustration of how one sample of overbank sediment (left)
represents many samples of active stream sediments (right).
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 195
between the W content in the overbank sediment samples
and the presence of known W occurrences in the bedrock,
and (4) the distributions of elements such as Be, Cr, Ni,
Rb and V characterize known geological formations in the
region.
In Greece, similar data on the representativity of over-
bank sediment, as those described for China, were obtained
by Demetriades and Volden [33]. They state that the element
content in only one sample of overbank sediment represent-
ing a large drainage basin is close to the median value for
the same element in several hundred samples of stream
sediments taken from small sub-catchments within the large
basin, see also p. 00.
In Belgium and Luxembourg (survey area 33,000 km2)
Van der Sluys et al. [46], produced a geochemical atlas
based on samples of overbank sediments at 66 sites located
in the apexes of catchments, which range in size from 60 to
600 km2, see also Refs. [18–21]. At each site an overbank
Fig. 13. Lead content in single samples of overbank sediment (left) and median
province, southern China. After Xiachu and Mingkai [44].
profile was dug near the river, and bulk samples were taken
at depths of 5–25 cm and, if possible, at 1.5–1.7 m. For most
sample sections evidence such as 14C dating and the absence
or presence of anthropogenic particles were used to
determine if the samples were from pre- or post-industrial
eras. Present-time active stream sediments were also
sampled. After drying, the samples were sieved to minus
0.125 mm fractions, which were analysed for the total
contents of 10 major and 11 trace elements. An example of
their maps is reproduced in Fig. 14.
The element contents in the lower overbank sediments
indicate the natural geochemical background, which varies
in a systematic way for several elements throughout the
survey area. This background reflects the composition of the
underlying bedrock.
The active stream sediments and the upper overbank
sediment have been polluted to a varying degree, and by
comparing trace element concentrations in these media
with the concentrations in the lower overbank sediment, the
degree of pollution could be assessed. From such data it is
clear that the most severe pollution occurs in the northern
part of Belgium, where the population is denser and
industry is more developed than elsewhere in the survey
area.
Xuejing and Hangxin published the most recent results of
a pilot study for the use of overbank sediment in China [47].
They selected 500 floodplains across the entire country
(9,600,000 km2) for sampling, each plain representing a
drainage basin in the order of 1000–6000 km2. At each plain
two samples were collected at depths of 0–25 and 80–100
cm, respectively. The samples were analysed for 71
elements. Statistical parameters for the analytical results as
well as maps for the distributions of Cu, Hg and Ni are
presented in the publication. Element contents in the widely
values per catchment for lead in stream sediments (right) in the Jiangxi
Fig. 14. Al, K, Sc and Si contents in overbank sediments in Belgium and Luxembourg. After Van der Sluys et al. [46].
B.Bølviken
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B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 197
spaced samples were compared with those from China’s
Regional Geochemical National Reconnaissance Program
(CRGNRP), which includes N1million samples of active
stream sediments. Selected results of these comparisons are
presented in Fig. 15, which shows that the geochemical data
generated from the wide-spaced sampling are strikingly
similar to those generated by the CRGNRP. A map of the
ratio between Hg contents in the surface samples and Hg in
the samples taken at depth (Fig. 16) demonstrates clearly,
that Hg (supposedly airborne) from industrial and urban
sources has polluted the eastern part of China. These results
have lead to the establishment of abatement strategies,
which will include monitoring of time trends in the pollution
by repeated sampling and analysis of overbank sediments
every 10 years.
Fig. 15. Distribution of copper in China. (A) Upper part: Averages from great n
Averages of restricted numbers of overbank sediment samples per catchment. Af
4. Summary discussion and conclusions
The characteristics of drainage regimes and sedimenta-
tion processes vary throughout the world, but it appears that
in most places overbank sediments are readily available and
very useful as a sampling medium in geochemical mapping,
even in polluted areas. In some places, such as in parts of
Britain and Mexico, as well as in heavily polluted areas in
the arctic, the geological and historical setting apparently
makes detailed studies necessary in order to obtain relevant
samples.
There is hardly an alternative sample type if the goal is to
detect both natural and polluted patterns at regional to
continental scales. Biological substances and materials such
as active stream sediments and stream waters may be
umbers of stream sediments within each catchment basin. (B) Lower part:
ter Xuejing and Hangxin [47].
Fig. 16. Mercury pollution as indicated by the ratio between the mercury
content in overbank sediments near the surface (0–25 cm) and that at depth
(80–100 cm) After Xuejing, and Hangxin [47].
B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199198
polluted to an unknown degree, while soils in addition have
undergone soil forming processes, jeopardizing a compar-
ison of the pollution of ancient and recent layers in vertical
sections. Lake sediments have some of the same properties
as overbank sediments [48], but are not easily inspected
before sampling. Furthermore, lake sediments are not
available in many areas due to the lack of suitable lakes.
Depth integrated samples of overbank sediments reflect
the composition of many current and past sediment sources
upstream of the sampling point, contrary to active stream
sediments, which normally are recent deposits originating in
a more restricted number of presently active sediment
sources. In most regions where tests have been performed,
overbank sediments are more representative of large parts of
drainage basins than are active stream sediments. Overbank
sediments can, consequently, be used to disclose main
regional to continental geochemical distribution patterns
with widely scattered sampling at low cost per unit area.
The stratigraphy of overbank sediments may in some
cases be complicated due to secondary processes. However,
in flood plains or old terraces along laterally stable or slowly
migrating channels it is normally possible to obtain recent
sediments near the surface and pre-industrial sediments at
depth. Simultaneous mapping of the composition of recent
and pre-industrial overbank sediments can normally be used
(1) in a characterization of variations in the natural
geochemical background as well as in a documentation of
the present state of pollution of some elements, and (2) as a
regional prospecting tool in natural as well as polluted
environments.
The composition of overbank sediment is also of great
interest to society in general since flood plains are very
important for agriculture, urbanisation and as sources for
drinking water.
Although vertical movements of easily soluble elements
between strata of overbank sediments have been reported,
the overall impression is that such chemical migration is not
a major problem in the use of overbank sediments in
geochemical mapping. However, great care should always
be taken in the sampling of overbank sediment. This is
particularly warranted in areas with severe pollution from
local sources.
In conclusion we emphasize that overbank sediments
represent a natural analogue to a bed-blended stockpile. This
follows from the documented empirical data and from
characteristic features of overbank sediments such as: (1)
They are build up from a succession of broadly similar
stacking and layering flooding events, (2) they are formed in
closely bracketed time intervals, and (3) each flood drains
the largest possible number of source locations within the
catchments.
Gy [4] has shown that industrially laid up stockpiles are
effective averages for very large lots. In principle the only
significant difference between an industrial stockpile and a
deposit of overbank sediment is that the first is man made,
while the other is natural. The degree of success in the
averaging process of this type of natural deposits has only
been summarised in this paper, and a future more in-depth
treatment is warranted.
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